Screw pumps

ABSTRACT

A screw pump of the type having at least two meshing screws with helical flights rotatable in, and having edge sealing with, bores formed in a casing, the casing being formed with one or more holes through its wall to communicate with the moving chambers between the screws located towards the discharge end of the pump, the position and size of the holes being such as to allow gradual collapse of cavities in the chambers.

United States Patent Inventor Stamford Robert Francis Vanderstegen- Drake Berkshire, England Appl. Nov 817,109 Filed Apr. 17, 1969 Patented Apr. 13, 1971 Assignee Plenty & Son Limited Newbury, Berkshire, England Priority Apr. 19, 1968 Great Britain 18,720-68 SCREW PUMPS 4 Claims, 2 Drawing Figs.

US. Cl 418/15, 418/197, 418/201 Int. Cl F04c l/10 Field of Search 103/ 126, 126 (F), 128; 230/143; 123/12 (BC); 91/84; 418/15, 197, 201

Primary Examiner-Mark M. Newman Assistant Examiner-Wilbur J. Goodlin Attorneys-Dowel] and Dowell, Arthur E. Dowell, Jr. and

Arthur E. Dowell ABSTRACT: A screw pump of the type having at least two meshing screws with helical flights rotatable in, and having edge sealing with, bores formed in a casing, the casing being formed with one or more holes through its wall to communicate with the moving chambers between the screws located towards the discharge end of the pump, the position and size of the holes being such as to allow gradual collapse of cavities in the chambers.

PATENTED APR 1 3 ISYI scnsw PUMPS DESCRIPTION OF INVENTION This invention relates to screw pumps of the type having at least two meshing screws with helical flights rotatable in and having edge sealing with, bores formed in a casing.

Hitherto cavitation problems have been encountered with pumps of this type due to the fact that if the speed of rotation of the screws are above a certain value cavities are formed in the fluid being pumped at the point where the fluid enters the moving spaces or chambers formed between the flights of the screws and the casing; the cavities being filled with air or gas. When the fluid is expelled at the other, or outlet, end of the screws the increased pressure at this end as compared with the inlet end, causes the pockets or cavities to implode i.e., collapse inwardly, resulting in noise and vibration.

This problem is much reduced with a screw pump of the type described in accordance with the invention in which the casing is formed with one or more holes communicating with the moving chambers between the screws. Thus fluid can be admitted to the chambers and the pressure therein thus gradually increased as the chambers approach the discharge end of the screws, causing the cavities to collapse slowly and without shock before the liquid is discharged.

The number and size of holes will depend upon the working conditions e.g. the designated speed of rotation, the fluid to be pumped and the'dimensions of the screws but for example with a screw diameter of 160 mm. two holes may be present each of a diameter of i9 mm.

The holes are preferably situated longitudinally within a distance which is less than the maximum diameter of the driving screw i.e. the screw which is driven and which acts to drive the screw or screws meshing with it. The holes are preferably situated to enter the bores of the casing at the point where the two screws come out of mesh with each other.

As the major portion of the screw length from the inlet end is not in communication with the fluid through the casing, proper scaling is maintained over this major length and it has been found that the capacity and air handling ability of a pump in accordance with the invention is not significantly altered as compared with conventional pumps.

An embodiment of a pump in accordance with the invention will now be described by way of example with reference to the accompanying drawing in which:

FIG. 1 is a side elevation, partly broken away, of the pump; and

FIG. 2 is a cross section.

The pump comprises a casing generally indicated at 2 having bores to accommodate three screws 4, 6 and 8 the central screw 6 being driven and acting to drive the idler screws 4 and 8 due to the interengagement of its helical flight 10 with the corresponding flights of the screws 4 and 8, the flight of the driven screw being convex and those of the idler screws being concave.

The casing has a sealing engagement with the edges of the helical flights of the screws and is formed both at the suction end, namely the end at the right-hand side of FIG. 1, and at the discharge end, namely the end at the left-hand side of FIG. 1, with ports for the inlet and discharge respectively of the fluid to be pumped, fluid being conveyed through the pump in chambers formed between the casing and the meshing flights of each pair of screws, 4, 6 and 6, 8 respectively.

In order to prevent the implosion i.e., inward collapsing of cavities which can be present in the liquid being conveyed through the pump by the screws, holes 12 are provided adjacent the discharge end of the pump and passing through the casing to communicate with the chambers carrying the liquid. In this way the pressure in the chambers is gradually increased as they approach the discharge end so that the cavities gradually collapse without a violent implosion, i.e. inward collapsing.

The holes are positioned as can be seen in FIG. 2 so as to enter the bores of the casing at the oint where the flights of the screws come out of mesh and e holes communicating with the chambers between the screws 6 and 8 are located on the opposite side of the casing to the holes communicating with the chambers formed between the screws 4 and 6.

The holes are all situated within a distance from the discharge end of the pump which is less than the maximum diameter of the screw 6, including its flight l0.

lclaim:

l. A screw pump of the type having at least two meshing screws with helical flights rotatable in, and having edge sealing with, bores formed in a casing, the casing being formed with one or more holes through its wall to communicate with the moving chambers between the screws located towards the discharge end of the pump, the position and size of the holes being such as to allow gradual collapse of cavities in the chambers.

2. A screw pump as claimed in claim 1 in which the holes are situated longitudinally from the delivery end of the bores in the casing within a distance which is less than the maximum diameter of the driving screw.

3. A screw pump as claimed in claim 1 in which the holes are positioned to enter the chambers in the casing at the point where any two screws come out of mesh with each other.

4. A screw pump as claimed in claim 1 having three screws in which the hole or holes communicating with the chamber between one pair of screws is located on the opposite side of the casing to the holes communicating with the chamber between the other pair of screws. 

1. A screw pump of the type having at least two meshing screws with helical flights rotatable in, and having edge sealing with, bores formed in a casing, the casing being formed with one or more holes through its wall to communicate with the moving chambers between the screws located towards the discharge end of the pump, the position and size of the holes being such as to allow gradual collapse of cavities in the chambers.
 2. A screw pump as claimed in claim 1 in which the holes are situated longitudinally from the delivery end of the bores in the casing within a distance which is less than the maximum diameter of the driving screw.
 3. A screw pump as claimed in claim 1 in which the holes are positioned to enter the chambers in the casing at the point where any two screws come out of mesh with each other.
 4. A screw pump as claimed in claim 1 having three screws in which the hole or holes communicating with the chamber between one pair of screws is located on the opposite side of the casing to the holes communicating with the chamber between the other pair of screws. 